422 THE DIGESTIVE APPARATUS IN MAMMALIA. 



tooth which has been worn, an external covering of enamel, and two circles, or 

 rather two irregular polygons, of central enamel circumscribing the two cavities. 

 In the superior molars, these bands of enamel represent a Gothic B, having a 

 small appendage on the loop nearest the entrance to the mouth. This figure is 

 modified in the teeth of the lower jaw, the enamel of the infundibuli being con- 

 tinuous, on the inner side, with the external enamel. The cement is extremely 

 abundant, and in the upper molars its total quantity nearly equals that of the 

 dentine ; it accumulates in the cuh-de-sac and on the external covering of enamel, 

 where it partially fills up the flutings on the faces of the crown. Prolonged 

 steeping of a molar tooth in hydrochloric acid, easily permits the isolation of 

 these elements. 



Owing to the arrangement above described, the section of an adult molar 

 tooth, naturally represented by the surface of friction (Fig. 284), exhibits, out- 

 wardly, a layer of cement ; next, the external enamel ; between this and the 

 central enamel, the dentine, always yelloAver, and sometimes even black in the 

 middle ; lastly, the enamel bands of the infundibuli, 

 ^'g- ^ - and the crusta petrosa filling them. As these enamel 



bands are much harder than the other substances, they 

 are worn more slowly, and stand out in relief on them. 

 The table of the tooth has also, for this reason, the 

 appearance of a veritable mill-stone, and is admirably 

 disposed for the trituration of those fibrous substances 

 on which the animal usually feeds. 



The follicle which develops these three elements of 

 J8 D E the molar tooth, has at the bottom an enormous papilla 



TRANsvKRSE SECTION OF A (jividcd luto scvcral lobes, which lie together for their 



HORSE S UPPER MOLAR , , , , i i i • , i ■ , , i , i 



TOOTH. whole length ; lodged m the mternal dental cavity, it 



A, External cement ; B, ex- gradually decreases, like the papilla in the other kinds 



ternal enamel ; c, dentine; of teeth, as the cavity bccomes diminished by the 



°e4d"c°rustTprrosa^' '"' formation of new dentine. Opposite to it are two long 



papillas, which occupy the enamelled infundibuli. 



"It was believed for a long time that the molars of Solipeds were all 



persistent teeth. This error, founded on the authority of Aristotle, was so 



deeply rooted, that although Ruini, towards the end of the sixteenth century, 



had discovered the existence of two temporary molars, Bourgelat did not believe 



it when he founded the French Veterinary Schools, and was only convinced 



when Tenon had proved by specimens, in 1770, that the first three in each 



arch are deciduous. 



" The replacement of these twelve molars is not at all like what happens with 

 the incisors. The molar of the adult grows immediately beneath the temporary 

 one, and divides its two fangs into four, until its body is reduced to a simple 

 plate and falls off, allowing the contracted summit of the permanent molar to 

 appear ; and this grows up until it is soon on a level with the others in the row, 

 " The first replacing molar is always a little more elongated than that which 

 it succeeds, and it most frequently expels at the same time the sup]plementary 

 molar ; so that if forty-four teeth be developed in the male Horse, it is very rare 

 that they are all present at the same time." 



Ruttimeyer has remarked that the tables of the first three molars are shorter 

 and wider in the Ass than in the Horse. The Gothic B the enamel forms is 

 consequently compressed, and the appendage to the anterior loop is wider in the 



